1. Field of the Invention
This invention relates to the field of junctions in semiconductive materials, and more particularly to junctions for use in photovoltaic detectors.
2. Description of the Prior Art
Infrared (IR) technology has received intensive research and development, from materials and devices through systems and applications. Imagery, communications, and target designation are important applications of this technology. While much progress has been made throughout the field, there is still great demand for better performance and reliability at low cost. One of the most crucial parts of the system which needs improvement is the focal plane. Any improvements in this area can ease the requirements and cost of the rest of the system.
The sensing volume of a p-n diode detector used for the focal plane is the depletion region between the n and the p layers. Any carriers that do not cross this region will not be counted. Thus, if light is incident on the p-n diode and all of this light is absorbed in the n or p region far from the junction, then no signal will be measured even though all of the light is absorbed because it is absorbed uselessly. To avoid this loss of signal, the top region, usually an n-region, of a homojunction diode is made thin so that most of the light is absorbed near or in the junction. But since light absorption is a function of the top region's thickness, the detected signal is also a function of this thickness. Non-uniformity of thickness then results in a non-uniformity of signal. Another complication associated with homojunction p-n diodes is that surface recombination can play an important role. Any variation in it will also mean non-uniformity of signal.
An atmospheric window for IR exists in the spectral range of 8 .mu.m to 13 .mu.m. An ideal IR array for use in this window should have a spectral response in the 8-13 .mu.m range with constant efficiency. Previous attempts have been made to use PbSnTe in a junction as a photo-voltaic detector in this spectral range. However, these attempts have not been successful in obtaining a spectral response much beyond 11.5 .mu.m.
To obtain longer wavelength spectral response in such a PbSnTe junction, the energy gap of the active semiconductive material must be reduced. However, prior art continuous growth techniques cannot accomplish this result because the required higher growth temperature increases the diffusion of the constituents. This causes the p-n junction to move away from the metallurgical junction, reducing the amount of light which is collected. According to the known art of epitaxy growth of semiconductive materials upon a substrate, material is deposited from environments upon the substrate by various techniques such as closed tube and open tube flow methods. Deposition occurs under continuous conditions, generally at elevated temperatures. When junctions are being grown, diffusion of constituents across the metallurgical junction causes the p-n junction to be located in one of the materials away from the metallurgical junction. When this occurs in photo-voltaic devices, the relative spectral response and quantum efficiency of the junction is reduced.